Stabilization of acylated or alkylated polyoxymethylenes



United States Patent )fifice 3,315,255 Patented Apr. 2, 1968 4 Claims.(03260-4575) The present invention relates to normally solid filmformingacylated and/or alkylated polyoxymethylenes of .high molecular weightwhich are stabilized by means of organic nitrogen compounds.

This application is a division of application Serial No. 31,259, asfiled May 24, 1960 now US Patent 3,296,194.

w,w'-DihydroXy-polyoxymethylenes of high molecular weight only show aslight terminal stability in their thermoplastic range between 170 and200 C. A degradation of the high-molecular chain molecules takes placeto give substantially lower molecular weights with formaldehyde beingsplit ofi mainly from the ends of the chains; a result is the loss ofthe valuable physical properties and the intrinsic viscosities fallsubstantially to values 0.4 (measured in dimethyl formamide at 150 C.).

It is already known to add hydrazines, hydrazides, phenols, aromaticamines, urea and t hiourea derivatives to thew,w'-dihydroxypolyoxymethylenes for improving the thermal stability (seeUS. patent specification No. 2,810,708, Belgian Patent 558,777 andBritish patient specification No. 748,856). It is true that thethermostability of the w,w'-dihydroxy-polyoxymethylenes is improved inthis way, but the degradation and the quantity of formaldehyde liberatedwhen processing at temperatures above 180 C. is still so considerablethat so far it has not been possible to effect a processing to highgradeplastics.

Further improvement in the thermostability of thew,w-dihydroxy-polyoxymethylenes can be obtained by the terminal hydroxylgroups of the polyoxymethylenes being acylated or alkylated. In thisway, the amount of formaldehyde split off from the ends of the chains attemperatures above 180 C. is substantially reduced. The amount offormaldehyde split off by thermal cracking of the molecule, which isincreased by traces of acids and impurities, and the degradationrecorded in this case, is however always still sufficient tosubstantialy impair the shaping and the physical properties ofpolyoxymethylenes of high molecular Weight. Thus, polyoxymethylenediacetates and diethers often show at 220 C. and within the first 5minutes, decomposition speeds which result in 0.3 to 0.4% offormaldehyde being split off per minute. At lower temperatures, forexample at 190 C., the products certainly split off a smaller quantityof formaldehyde, but the moulded elements manufactured at thistemperature are however permeated by gas bubbles. Furthermore, on beingmelted, particularly in the presence of air, the molecule is degraded toabout /3 of the original molecular weight. As a consequence, themechanical property values, such as impact bending strength andtoughness, are appreciably lowered.

In French specification 1,131,939 there are disclosed polyoxymethyleneswhich have added thereto antioxidants such as phenyl-fl-naphthylamine or2,6-tertiarybutyl-p-cresol. These additives have the drawback that theyeither give rise to a strong discoloration of the compositions or thatthey do not stabilize compositions to a degree which is required by thepractice. In French specification 1,179,857 there are disclosed acylatedor alkylated polyoxymethylenes of high molecular weight which arestabilized by addition of polyamides.

It has now been found that the terminal stability of normally solidfilm-forming acylated or alkylated polyoxymethylenes can be essentiallyimproved by adding to said polyoxymethylenes at least one organicnitrogen compound out of the following groups of compounds:

(1) Amines and hydrazines of the general formulae:

in which X represents a radical comprising an ester group, ether andthioether group, carbonamide group, urethane group, acetal group ornitrile group, bonded to the nitrogen atom by way of an aliphaticradical, or an organic radical, preferably a hydrocarbon radical bondedto the nitrogen atom by way of a Si atom; and R R and R each representaliphatic, cycloaliphatic or araliphatic radicals or like or differentsubstituents X, it being possible for two of the substituents R R R alsoto jointly represent chain members of a ring system which can containfurther hetero atoms and/or double bonds.

(2) Amines and hydrazines of the general formulae:

wherein R R R and R represent aliphatic, cycloaliphatic and araliphaticradicals, and in case of Formula 111 two of the substituents R R R canjointly be chain members of a ring system which if necessary can containfurther hetero atoms, and the sum of the carbon atoms of allsubstituents R R R and R is greater than 12, preferably 12 to 40.

(3) Salts of inorganic or organic bases and dithiocarbamic acids.

(4) Aromatic and cycloaliphatic primary, and tertiary diamines, whichare alkylated in positions to the amino group.

(5) The fifth group of the stabilizers according to the inventioncomprises the following classes of compounds: 1,6-tetra-substitutedhydrazodicarbonamides, hydrazodicarboxylic acid esters with monohydricaliphatic, cycloaliphatic or araliphatic alcohols, which esters containup to 40 carbon atoms, l-substituted hydrazine mono-carboxylic acidesters, l-su'bstituted semi-carbazide and thiosemicarbazidemonocarboxylic acid esters, 1,4-substituted semi-carbazides andthio-semi-carbazides, monosubstituted urethanes and thiourethanes,allophanic acid esters, biuret derivatives, N-substituted melaminescontaining NH groups, substituted guanidines, formamidines, and amidinesas well as iminoethers, the preferred substituents thereon being alkyl,cycloalkyl, aryl and aralkyl groups having together up to 40 carbonatoms.

(6) Aldehydes of tertiary aromatic amines or their functionalden'vaties, such as for example acetals, azines, hydrazones, oximes,semi-carbazones, Schiffs bases and others.

Examples of suitable stabilizing agents of the recited group 1 ofcompounds are the following:

(In) Esters of silicic, phosphoric, carbonic acid or of saturated orunsaturated aliphatic, cycloaliphatic, araliphatic or aromaticcarboxylic acids (having up to 25 carbon atoms) with tertiary amines orhydrazines which are substituted at least one nitrogen atom by at leastone hydroxalkyl group, the other substituents being alkyl,

secondary the ortho 3 cycloalkyl or aralkyl radicals (the sum of thecarbon atoms in said amines or hydrazines being up to 54).

These compounds may be represented by way of example by the followingformulae:

.n which X; stands for Siz, :1: or O=C=, A is a bivalent aliphaticradical having 2 to 3 carbon atoms n is 'a whole integer from 2 to 4corresponding to the valence of X, R is a like or different monovalenthydrocarbon radical (alkyl having 1 to 20 carbon atoms, cycloalkyl suchas cyclohexyl, aralkyl such as benzyl).

in which X stands for the radical: -A.O.(A.O-) .CO. R7 (A having thesame meaning as above, n Standing for a whole integer from O to 3 and Rfor alkyl having 1 to 20 carbon atoms, cycloalkyl such as cyclohexyl,aralkyl such as benzyl and aryl such as phenyl, tolyl) R and R arealkyl, cycloalkyl, aralkyl (as nearer defined above) or X and R and Rtogether may stand for members necessary to close a heterocyclic ringsuch 'as a morpholine, thiomorpholine ring.

alkylene having 1 to 12 carbon atoms, phenylene, cyclohexylene).

has the same meaning as above and R R in which X hydrocarbon radicals asindiand R are monovalent cated above or X Among the compoundscorresponding to the last three formulae those are preferred whichcontain more than 12, preferably to 36 carbon atoms.

(1b) Ethers and polyethers of saturated or unsaturated aliphatic,cycloaliphatic, araliphatic or aromatic hydroxy compounds includingpolyglycols (having up to carbon atoms) with tertiary amines orhydrazines which are substituted by at least one hydroxyalltyl group(having preferably 2 to 3 carbon atoms) the other substituents beingalkyl, cycloalkyl or aralkyl radicals (all substituents having up to 54carbon atoms).

(10) Compounds of the Formulae l and II in which X stands for one of theradicals:

l R13 R13 R13 wherein R and R stand for hydrogen, aliphatic,cycloaliphatic or araliphatic radicals having up to 26 carbon atoms.

(1b) The reaction products of isocyanates with hydroxy-alkylatedtertiary amines and hydrazines corresponding to Formulae I and II inwhich X stands for the radical A.O.CO.NH.R wherein A has the samemeaning as above and R stands for a monovalent hydrocarbon radical(alkyl having 2 to 18 carbon atoms, and such as phenyl, tolyl, naphthyl,aralkyl such as benZyl, cyclo'alkyl such as cyclohexyl), furthermore thereaction products of 1 mole of an aliphatic, cycloaliphatic oraraliphatic diisocyanate (having 6 to 10 carbon atoms) with 2 moles of amonohydroxyalkylated tertiary amine or hydrazine as defined above.

(1e) Acetals of the above defined hydroxyalkylated tertiary amines andhydrazines with aliphatic aldehydes having 1 to 7 carbon atomscorresponding to the Formulae I and II wherein X stands for the radical-A.O.B.O.'R wherein A has the same meaning as above, B stands for abivalent aliphatic radical having 1 to 7 car bon atoms and R stands foralkyl (having 1 to 20 carbon atoms), aralkyl cycloalkyl.

(1f) Compounds of the Formulae I and II wherein X stands for theradicals -CH -CH CN and R standing for alkyl having preferably 1 to 2carbon atoms or in which X stands for the two free valencies of theradical being saturated by the radicals resulting by subtraction of Xfrom the compounds of Formulae I and 11. Those compounds are preferredwhich contain only one silicon atom.

Specific examples of stabilizers in the group according to the inventionare N,N'dibutyl-trimethylsilylarnine, N methyl Noctadecyl-trimethylsilylamine, N,N,N', Ntetra-n-butyl-dimethylsilyldiamine, dimethylsilylbis-N,N-stearyl-methylamine, while among the series of the ester amines,there are mentioned esters of silicic, phosphoric, carbonic andcarboxylic acids, such as for example esters of acetic acid, propionicacid, benzoic acid, malonic acid, succinic acid, phthalic acids withhydroxyethylated or hydroxypropylated tertiary aliphatic,cycloaliphatic, araliphatic or heterocyclic amines, or the correspondinghydrazines, such as for example the esters of triethanolamine, N-methyldiethanolamine, N,N-di-nbutyl-ethanolamine and N,N stearyl methylethanolamine (N-methylN-stearyl ethanolamine acetate or propionate, orthe corresponding n-propanolor isopropanol amine derivatives, N-lauryl Nbutyl ethanol (propanol) amine acetate or propionate,N-cyclohexyldiethanolamine diacetate or propionate,N-hydroxymethylmorpholine acetate or propionate,N-hydroxyethylpiperidine acetate or propionate,N,N-dibenZyl-N,N'-difi-acetoxyethyl hydrazine, N,N'-dicyclohexylN,N'-di-fiacetoxyethyl-hydrazine). Moreover, reaction products from thesaid acids with hydroxyalkyl amines containing ether groups in the alkylchain which can for example i be obtained by the action of ethyleneoxide or propylene oxide on primary and secondary aliphatic,cycloaliphatic and araliphatic amines. Other ether amines of this groupare the reaction product of phenol, guiacol or resorcinol withN,N-diethylaminoethyl chloride; also to be mentioned are the reactionproducts of monoisocyanates and polyisocyanates (hexyl-, cyclohexyl-,phenyl-, benzyl isocyanate, tetra-, hexamethylene, 1,4-cyclohexyldiisocyanate, 4,4'-dicyclohexyl methane diisocyanate) with aminoalcoholssuch as for example N-methyl-diethanolamine or N,N-dibutylethanolamineor the corresponding propanolamine derivatives, whereby thepolyisocyanates are reacted with mono-alkanolamines, the proportionsbeing such that the reaction products do not contain free hydroxy orisocyanate groups.

Examples of compounds of the second group are N,N- diethyl stearylamine,N,N-dibutylstearylamine, N,N'-dibenzyl-N,N'-diethyl hydrazine, Nstearylmorpholine, N- lauryl-piperidine, N,N-dipropylstearylamine.

Examples of stabilizers of the third group for use according to theinvention are salts of N,N-dibutyl-, N- methyl-N-stearyl-,N-methyl-N-lauryl-, N butyl N- stearyl-, N-isopropyl-N-butyl-,N-cyclohexyl-N-butyl-, N- cyclohexyl-N-benzyl-dithiocarbamic acids withmetals (such as sodium, potassium, lithium, calcium, strontium, barium,magnesium, zinc, iron, cobalt, nickel) ammonia or amines, such as forexample with dibutyl amine or methyl stearylamine.

Examples of the stabilizers of the fourth group are2,3,5,6-tetraethyl-p-phenylene diamine, 2,3,5,6tetraisopropyl-p-phenylene diamine, 2,3,5,6 tetra tertiarybutyl-p-phenylene diamine, 3,3',5,5'-tetraisopropyl 4,4- diaminodiphenylmethane and the corresponding perhydrogenated, peralkylated derivatives.

Examples of the stabilizers of the fifth group are N,N, N"",N""-tetra npropyl hydrazodicarbonamide, N, N,N"",N"-tetra n butylhydrazodicarbonamide, hydrazodicarboxylic acid dipropyl ester,hydrazodicarboxylic acid dibutyl ester, N,Ndimethylhydrazine carboxylicacid lauryl ester, reaction products of hexamethylene diisocyanate orhexamethylene diisocyanate with monoand polyglycols or mercaptans,1-pheny1-4-dibutylsemicarbazide, 1,4-diphenyl thiosemicarbazide,trimethylol-melamine tri-n-butyl ether, N phenyl-N',N',N",N"-tetramethyl guanidine, N,N'-N"-triphenyl guanidine, N, N-N"-triphenylguanidine, N-phenyl-N,N'-dimethylformamidine, N,N'-diphenyl benzamidineand benziminoethyl ether.

Examples of the stabilizers of the sixth group are 4-diethylaminobenzaldehyde, 4 dimethylaminobenzaldehyde azine (reactionproduct of 2 mols of the aldehyde with 1 mol of hydrazine), 4diethylaminobenzaldehydephenyl hydrazone, 4 diethylaminobenzaldehyde-phenyl hydrazone and 4 diethylaminobenzaldehyde phenylsemicarbazone.

The aforesaid compounds are suitable for stabilizing normally solid,film-forming polyoxymethylenes of high molecular weight having terminalacyl or ether groups, such as acetyl, propionyl, stearoyl, benzoyl, acylgroups derived from cyclohexyland phenyl acetic acid (that is to sayacyl groups derived from aliphatic, cycloaliphatic, araliphatic andaromatic carboxylic acids), and alkyl ether groups having preferably 1to 4 carbon atoms. For producing these products the normally solidpolyoxymethylenes may be acylated by means of acetic acid, propionicacid, benzoic acid or other carboxylic acids and/or may be alkylatedwith the aid of ortho-esters, such as for example orthoformic acidesters. Such acylated and/or alkylated polyoxymethylenes of highmolecular weight are for example described in Belgian patentspecification 583,933, and in the U.S. patent specifications 3,170,896and 3,046,251, U.S. patent application Ser. No. 21,855, filed Apr. 13,1960 now U.S. Patent No. 3,193,531.

The stabilizers according to the invention are added to thepolyoxymethylenes stabilized at the terminal groups in quantities ofabout 0.01 to 4 percent by weight, advantageously 0.5 to 3 percent byweight, prior to the processing and shaping, it being possible for thestabilizers to be used singly or in any desired mixture. It is moreoverpossible to use then in combination with known antioxidants, such as forexample phenols, sulfur-containing compounds such asmercaptobenzthiazole, and

also with fillers, such as carbon black, plasticizers, lubricants,inorganic and organic pigments and other additives.

The stabilizers or mixtures of these stabilizers can be added insubstance to the recited acylated or alkylated polyoxymethylenes bymilling or kneading, but they are advantageously dissolved in a solventand sprayed on to the product or intimately mixed with excess solventstogether with the polyoxymethylene stabilized at the terminal groups,the solvent thereafter being evaporated.

The stabilizers according to the invention permit more especially aprocessing of the high molecular weight polyoxymethylenes stabilized atthe terminal groups in accordance with the injection moulding process,without the molecular weights of the said polyoxymethylenes falling intoranges which are characterized by brittleness and low strength values.

The stabilizing action of the stabilizers according to the invention isto be shown by reference to the following comparison: Whereas apolyoxymethylene diacetate with an intrinsic viscosity of 0.85 (measuredat a 0.5 percent solution in dimethyl formamide at 150 C.) on beingmelted while in contact with air at 200 C. during a heating period of 2minutes, changes into a thinly liquid melt which, after cooling, yieldsa brittle film which breaks easily the said film consisting ofpolyoxymethylenes which only still have an intrinsic viscosity of 0.2,the same sample with the same intrinsic viscosity, to which for example0.5 percent of triethanolamine triacetate, N,N-dibutyl ethanolamineacetate or N-methyldiethanolamine diacetate are added, produces anintrinsic viscosity of 0.59 to 0.65 when treated in the same manner; thefilms thereby obtained are tough, pliable and elastic.

If the same experiment is carried through while using the same amount ofa high molecular weight polyamide (produced from e-caprolactam), theviscosity value drops to 0.42.

By the addition of the stabilizers according to the invention, themolecular weight ranges with the valuable physical properties(corresponding to intrinsic viscosities of 0.55 to 3.0 as measured at0.5 percent solution in dimethylformamide at 150 C.) are substantiallymaintained at the high processing temperatures.

In the following examples, the parts indicated are parts by weight.

Example 1 'Each batch comprising 10 parts of high-molecular acetylatedpolyoxymethylene having an intrinsic viscosity of 0.85 (measured at 150C. at a 0.5 percent solution in dimethyl formamide), is suspended in 60to parts by volume of acetone and the mixture has added thereto 0.4percent of beeswax and one of the following stabilizers in quantities of2 percent (:02 part): triethanolamine triacetate,N-methyl-diethanolamine diacetate or dipropionate,N,N-dibutyl-fi-ethanolamine acetate o propionate,N-methyl-N-stearylamino-propionitrile, silicicacid-tetra-(fl-N-dibutylaminoethyl) ester,dimethylsilylbis-(N-butylamine), phosphoricacid-tris-(B-N-dibutylaminoethyl)-ester, N,'N-dibutyl-N,N-di--acetoxyethyl-- hydrazine, N-methyl-N-stearyl-methylstearyl ammoniurndithiocarbamate, dimethylstearylamine, trimethyl stearylammoniumacetate, 2,3,5,6-tetraethyl-p-phenylene diamine,1,6-tetra-n-propyl-hydrazodicarbonamide, 1,4-diphenyl thiosemicarbazide,N,N-dimethylhydrazine carboxylic acid lauryl ester,p-dimethylaminobenzaldehyde phenyl hydrazone, fl-diethylaminoethylphenyl ether or 3,3',5,5'-tetraisopropyl-4,4'-diamino-diphenylmethane.

While stirring, the acetone is slowly evaporated, the polyoxymethylenemixture is shaken for 10 minutes, thereafter dried and Once againthoroughly mixed. Two parts of the unstabilized speimen and each of thestabilized specimen are melted in air at 200i1 in a thin layer andquickly cooled after 2 minutes. Whereas the melt of the unstabilizedspecimen splits off formaldehyde to an appreciable degree and the meltviscosity rapidly falls, the stabilized specimens show only a slight orno formaldehyde vapour pressure and remain viscous. In contrast to theunstabilized comparison specimen, they are tough and elastic aftercooling. The following table shows the extreme drop in the intrinsicviscosity of the nodiphenylmethane.

m+ (inherent viscosity) measured at a. 0.5 percent solution in dimethylformamide at 150 The polyoxymethylene referred to above is produced asfollows:

For the production of monomeric formaldehyde, para formaldehyde issubjected to thermal decomposition. The monomeric formaldehyde is mixedin the pyrolysis vessel with pure dry nitrogen and thereafter conductedthrough an extensive cooling system at a temperature of 20 C. In asecond cooling system at a temperature of 85" C., the formaldehyde isliquefied and supplied to the polymerization vessel. The polymerizationvessel is provided with an inlet pipe for formaldehyde, a mechanicalstirrer and a gas-outlet pipe. Disposed in the polymerization vessel are1000 parts by volume of anhydrous toluene, to which are added 0.035parts by weight of tetramethyl urea, dissolved in 3 parts by volume ofanhydrous toluene. The formaldehyde is added dropwise over a period of 2/2 hours and while stirring into the reaction medium cooled to 20 C.,whereby polymerization takes place. After stirring for another hour at20 C., the polymerization product is suction filtered. There is obtaineda pure White high molecular weight polyoxymethylene, which is extractedby stirring twice with methanol and twice With acetone. The product isthereafter dried in vacuo for 4 hours at 60 C. The yield is 118 g.

The polyoxymethylene thus obtained is acetylated according to thefollowing prescription:

20 parts of the recited polyoxymethylene are treated with 400 parts ofacetic anhydride, 20 parts of phenyl isocyanate (or parts oftolyl-2.4-diisocyanate) and 0.7 I

part of sodium acetate in a nitrogen atmosphere for hours at 139 to 140C. The acetylated polyoxymethylene is filtered off from the cooledreaction solution, freed from acetic anhydride by washing several timeswith acetone and methanol, freed from traces of sodium acetate bywashing with water, and dried after a further treatment with acetone.

Example 2 In a manner analogous to Example 1, 2 percent oftriethanolamine triacetate are admixed with an intrinsic viscosity of1.2 (measured at a 0.5 percent solution in dimethyl formamide at 150 C.)and the thermostability of the specimen is determined at 222 C. under anitrogen atmosphere. Whereas the unstabilized specimen has already lost5 percent of formaldehyde after minutes and 16 percent thereof after 120minutes, the stabilized specimen only loses 1 percent of formaldehyde in20 minutes and only 5 percent after 120 minutes.

Example 3 An acetylated polyoxymethylene of high molecular weight,having an intrinsic viscosity of 0.95 (measured at a 0.5 percentsolution in dimethyl formamide at 150 C.), is mixed in accordance withExample 1 with 1.2 percent of N,N-dibutyl-anunonium dithiocarbamate and0.5 percent of beeswax and the thermostability of the specimen isdetermined at 222 C. Whereas the unstabilized specimen has already splitoff 5.4 percent of formaldehyde after 20 minutes and 14.5 percent after120 minutes, the stabilized specimen loses only 3 percent offormaldehyde in 20 minutes and only 4.6 percent after 120 minutes.

The polyoxymethylene may be prepared by-polymerizing substantiallyanhydrous formaldehyde in an inert solvent while using aluminum oxide ascatalyst (compare French specification 1,226,239). The acctylation maybe carried through according to the prescription of Example 1 with thevariation that used as catalyst.

Example .4

A high molecular polyoxymethylene diacetate may be prepared according tothe data of Example 3 and has an intrinsic viscosity of 0.75 at 150 C.at a 0.5 percent solution in dimethyl formamide, is mixed according toExample 1 with 1.5 percent of 1,6-tetrapropyl hydrazodiand the thermocarbonamide and 0.5 percent of beeswax stability of the specimen isdetermined at 222 C. under a nitrogen atmosphere. Whereas theunstabilizecl specimen has already lost 12 percent of formaldehyde after20 minutes and 18 percent after 120 minutes, the stabilized specimenloses 6 percent of formaldehyde in 20 minutes and 10 percent after 120minutes.

Example 5 An ethylated high-molecular polyoxymethylene as ob tainedaccording to Example 1 of Belgian specification 583,593 is mixedaccording to Example 1 with 1.5 percent ofN-methyl-N-stearyl-methylstearyl-ammonium dithiocarbamate and thethermostability is determined at 222 C. under nitrogen. Whereas theunstabilized specimen has already lost 37 percent of formaldehyde after20 minutes and 79 percent thereof after 120 minutes, the stabilizedspecimen loses only 9 percent of formaldehyde in 20 minutes and 26.6percent after 120 minutes.

Example 6 An acetylated polyoxymethylene is used which has an intrinsicviscosity of 1.0 (measured at 0.5 percent solution in butyrolactone at150 C.). parts of the polyoxymethylene diacetate are suspended in asolution of 0.8 part of bis-thiourethane obtained from 1 mol ofhexamethylene diisocyanate and 2 mols of dodecyl mercaptan and 0.3 partof beeswax in acetone. Thereafter, the acetone is completely evaporatedin vacuo and the specimen is finally dried at 60 C. in vacuo. Thethermostability is determined at 222 C. under nitrogen. Whereas anunstabilized comparison substance has already split off 8 percent offormaldehyde after 20' minutes and 20 percent after minutes, the loss offormaldehyde with the stabilized specimen after 20 minutes is only 4percent and after 120 minutes it is only 7 percent.

Example 7 The procedure indicated in Example 6 is followed and I apolyoxymethylene diacetate is used which has an intrinsic viscosity of1.8 (measured at a 0.5 percent solution in butyrolactone at C.). Theadditives admixed in. acetone solution with the polyoxymethylenediacetate amount to 0.7 percent of N-methyl-N-stearyl-methylstearylammonium dithiocarbamate and 0.3 percent of beeswax. The measurements ofthe thermostability carried out under a nitrogen atmosphere at 222 C.show that an unstabilized comparison specimen splits off 14 percent offormaldehyde after 20 minutes and 37 percent thereof after 120 minutes,whereas the stabilized ene diacetate only loses 2 percent offormaldehyde after 20 minutes and only 7.5 percent thereof after 120minutes.

Example 8 diisopropyl carbondiimide is polyoxymethylviscositty of 0.72(measured in butyrolactone at 150 C.). The additives admixed with thepolyoxymethylene diacetate in acetone solution comprise 1.5 percent ofN- methyl N stearyl-methylstearyl ammonium dithiocarbamate and 0.3percent of beeswax. The thermostability measurements conducted under anitrogen atmosphere at 222 C. show that an unstabilized comparisonspecimen splits off 10 percent of formaldehyde after 20 minutes and 21.5percent thereof after 120 minutes, while the stabilized polyoxymethylenediacetate loses only 2.5 percent of formaldehyde after 20 minutes andonly 9 percent after 120 minutes.

Example 9 The procedure is as indicated in Example 6 and apolyoxymethylene diacetate is used which has an intrinsic viscosity of1.1 (measured at 0.5 percent solution in butyrolactone at 150 C.). Theaddtivies admixed with the polyoxymethylene diacetate in acetonesolution comprise 2 percent of 2,3,5,6-tetraethyl-p-phenylene diamineand 0.5 percent of beeswax. The thermostability measurements carried outunder a nitrogen atmosphere at 222 C. show that an unstabilizedcomparison specimen splits off 6 percent of formaldehyde after 20minutes and 13 percent thereof after 120 minutes, while the stabilizedpolyoxymethylene diacetate loses only 2 percent of formaldehyde after 20minutes and only 5 percent after 120 minutes.

Example The procedure is as indicated in Example 6 and apolyoxymethylene diacetate is used which has an iutrinsic viscosity of1.0 (measured at 0.5 percent solution in butyrolactone at 150 C.). Theadditives admixed with the polyoxymethylene diacetate in acetonesolution comprise 2.5 percent of diethyl stearylamine and 0.4 percent ofbeeswax. The thermostability measurements carried out under a nitrogenatmosphere at 222 C. show that an unstabilized comparison specimenalready splits off 10 percent of formaldehyde after minutes and percentthereof after 120 minutes, while the stabilized polyoxymethylenediacetate loses only 3 percent of formaldehyde after 20 minutes and only8 percent thereof after 120 minutes.

Example 11 The procedure is as indicated in Example 6 and apolyoxymethylene diacetate is used which has an intrinsic viscosity of1.4 (measured in butyrolactone at 150 C.). The additives admixed withthe polyoxymethylene diacetate in acetone solution comprise 2 percent of4-diethylaminobenzaldehyde phenol -hydrazone and 0.4 percent of beeswax.The thermostability measurements carried out in a nitrogen atmosphere at222 C. show that an unstabilized comparison specimen already splits off5 percent of formaldehyde after 20 minutes and 24.5 percent thereofafter 120 minutes, whereas the stabilized polyoxymethylene diacetateonly loses 2 percent of formaldehyde after 20 minutes and only 6 percentthereof after 120 minutes.

As to the compounds of group 3 of the present specification these maycorrespond to the general formula:

wherein the radicals denoted R are like or different monovalentaliphatic radicals (having 1 to 20 carbon atoms), cycloaliphaticradicals (such as cyclohexyl), araliphatic radicals (such as benzyl),and both radicals may form members of a heterocyclic ring (such as ofpiperidine or morpholine), Me stands for a metal or an amine asindicated above and n for a Whole number corresponding with the valencyof Me. As salt forming bases there are preferred secondary amines whichare substituted by aliphatic. araliphatic or cycloaliphatic radicalshaving preferably a total of 8 to 40 carbon atoms.

What we claim is:

1. A composition comprising a thermally stabilized, normally solid,synthetic high molecular weight polyoxymethylene selected from the groupconsisting of acylated and alkylated polyoxymethylenes and a Stabilizingamount of a compound of the formula:

ammonium and H R l l wherein each R is a monovalent substituent selectedfrom the group consisting of aliphatic and araliphatic, the total numberof carbon atoms in the R substituents being from 8 to 40, and n is awhole number corresponding to the valency of Me.

2. A composition according to claim 1, wherein said compound is presentin an amount of 0.01 to 4 percent by weight based on the weight of saidpolyoxymethylene.

3. A composition according to claim 1, wherein said compound isN-methyl-N-stearyl-methylstearyl ammonium dithiocarbamate.

4. A composition according to claim 1, wherein said compound isN,N-dibutyl-ammonium dithiocarbamate.

References Cited DONALD E. CZAJA, Primary Examiner. LEON I. BERCOVITZ,Examiner. H. E. TAYLOR, Assistant Examiner.

1. A COMPOSITION COMPRISING A THERMALLY STABILIZED, NORMALLY SOLID,SYNTHETIC HIGH MOLECULAR WEIGHT POLYOXYMETHYLENE SELECTED FROM THE GROUPCONSISTING OF ACYLATED AND ALKYLATED POLYOXYMETHYLENES AND A STABILIZINGAMOUNT OF A COMPOUND OF THE FORMULA